The United States Government and the governments of other countries/regions have installed and continue to maintain satellite constellations to provide location determining capability in their respective countries/regions. The Global Positioning System (GPS) is the US version of such Global Navigation Satellite System (GNSS). Throughout this disclosure, the generic term GNSS, the specific term GPS, or the combination GPS/GNSS may be used and such references shall refer to any such system, including GPS, GLONASS (Russian), Galileo (European), Indian Regional Navigation Satellite System (IRNSS), BeiDou-2 (Chinese), or other such comparable system.
Accordingly, many modern electronic and consumer devices include a GNSS receiver that can determine the absolute position of the device (in latitude, longitude, and altitude relative to a global coordinate system) via the GNSS system. GNSS receivers determine their position to high precision (within a few meters) by receiving time signals transmitted along a line-of-sight by radio (e.g., RF signals) from each “visible” GNSS satellite. Receipt of a sufficient number of signals (e.g., four or more) also allow the electronic receivers to calculate the current local time to high precision, which allows for time synchronization without the use of costly high precision oscillators.
One problem is that GNSS systems require a direct path (from each satellite used in a location and time synchronization solution) to the GNSS receiver in order to compute an optimum solution. In traditional GNSS systems, at least three simultaneous direct path signals from a corresponding number of space vehicles must be received to compute a solution if absolute GPS time is known at the receiver. However, rarely does the receiver have access to a high precision time source (e.g., a synchronized and highly precise oscillator), in which case four simultaneous GNSS signals may be required to provide for determination of placement in three dimensions and to correct for any timing bias at the receiver.
In outdoor environments, where there are no obstacles (e.g., on the open highway) this is easily realized. Further, in some light building structures (e.g., residential houses composed largely of wood) the GNSS signals are (1) only mildly attenuated, so that the received signal strength is above the sensitivity of the GNSS receiver, and (2) have an ideal, direct, and thus un-delayed path from the satellite to the receiver as seen in FIG. 1. In this case the signal rays are undistorted in time; that is, they have the same propagation time as a true line-of-sight (LOS) path and they can be used to provide an accurate location for the receiver.
However, in some environments such as heavy urban environments where a building may be embedded among a cluster of buildings, a direct signal ray may not exist from all of the requisite number satellites to determine a location and provide time synchronization. FIG. 2 shows a receiver located at a corner of a floor of a building, the building being in a homogenous cluster of buildings of equal size and spacing for illustration purposes.
In this case the receiver has a direct LOS signal from SV1 on the side with the least obstruction to the satellite. On the side facing SV2, the direct ray is attenuated below the sensitivity of the receiver. There is also a reflected ray from SV2 which necessarily is an elongated ray (because the distance traveled by the ray is greater than the distance between SV2 and the receiver. The elongated ray will introduce an error into the location estimate.
Present technology provides no solution for GNSS location in a cluster of buildings. Rather the locations of points to be located are done via manual and physical survey methods, which are laborious and costly.